The present invention relates to a method of making a corrosion-proof and corrosion-proofing layer on the inside surface of steel pipes.
Corrosion-proofing of water pipes by means of a cement mortar lining has been known for many years. This lining is usually applied by means of centrifugal force action. The resulting lining has, for instance, a water-to-cement ratio of 0.4 at a mixing ratio of sand to cement of approximately 2.5. The German Industry Standard, laid down in Paper W 342 of December 1978, sets forth requirements and test particulars for such a material, particularly when used in combination with drinking water.
One may use other methods of applying such a cement lining; decisive is that such a resulting cement mortar lining provides sufficient protection against corrosion. This is, indeed, true for most types of water; and pipe lines can in that manner be deemed completely corrosion-proof for most cases. However, water rich in carbon dioxide; industrial, acid-rich waste water; or salt water which contains additives being particularly aggressive to cement (e.g., magnesium salts or sulfates); all will corrode the cement and will slowly destroy the protective properties of steel pipes. The corrosion process involves, in particular, gradual dissolving of the calcium so that the mortar structure is disturbed and destroyed. This phenomenon was particularly observed where partially decalcinated mortar was permitted to dry. Temporary dryness is inevitable when, for instance, repairs and other maintenance is required. Here then, the mortar will loose directly some of its sand. Repetition of the wet-dry situation will gradually destroy the protection.
It has been suggested to improve the corrosion resistance of such mortar by adding, for example, organic polymer in a watery dispersion. However, a centrifugal application of the substance to a rotating pipe as per German paper W 342 mentioned above is not feasible because the synthetic dispersed in water will be removed by the centrifugal action.
German printed patent application No. 26 20 669 suggests casting the cement mortar against the inside wall of a non-rotating pipe by means of air pressure and under utilization of a rotating disk or distribution head. The mortar contains a synthetic dispersion, particularly for ensuring adhesion of the fresh mortar in the wall. This method, however, requires that, subsequently, the surface of the mortar layer be smoothed. Moreover, the material itself, as applied, is not adequately compacted. In the case of significant porosity with little or no compression, all reactions tending to dissolve the calcium are actually enhanced. A mortar blended with the dispersion of a synthetic will still decalcinate as fast as it would without the additive, but resistance against mechanical wear was noticably improved. See, for example, "MANNESMANN FORSCHUNGSBERICHTE" 770/1978, pp. 448-459.
It is a surprising discovery that significant compression of a synthetically alloyed mortar improves greatly its corrosion resistance, whereby particularly the strength of the mortar as well as its decalcination are involved.